Magnetic tape with particular refractive index characteristics, magnetic tape cartridge, and magnetic tape apparatus

ABSTRACT

Provided are a magnetic tape including: a non-magnetic support; and a magnetic layer including ferromagnetic powder and a binding agent on the non-magnetic support, in which a total thickness of the magnetic tape is equal to or smaller than 5.30 μm, the magnetic layer has a servo pattern, a center line average surface roughness Ra measured on a surface of the magnetic layer is equal to or smaller than 1.8 nm, and an absolute value ΔN of a difference between a refractive index Nxy of the magnetic layer, measured in an in-plane direction and a refractive index Nz of the magnetic layer, measured in a thickness direction is 0.25 to 0.40, a magnetic tape cartridge and a magnetic tape apparatus including this magnetic tape, a magnetic tape cartridge and a magnetic tape apparatus including this magnetic tape.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 16/727,181 filed Dec. 26, 2019, which claims priority under 35 U.S.C 119 to Japanese Patent Application No. 2018-246871 filed on Dec. 28, 2018. The above applications are hereby expressly incorporated by reference, in their entirety, into the present application.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a magnetic tape, a magnetic tape cartridge, and a magnetic tape apparatus.

2. Description of the Related Art

Magnetic recording media are divided into tape-shaped magnetic recording media and disk-shaped magnetic recording media, and tape-shaped magnetic recording media, that is, magnetic tapes (hereinafter, simply referred to as “tapes”) are mainly used for data storage such as data back-up and archive. The recording of information on a magnetic tape is normally performed by recording a magnetic signal on a data band of the magnetic tape. Accordingly, data tracks are formed in the data band.

An increase in recording capacity (high capacity) of the magnetic tape is required in accordance with a great increase in information content in recent years. As means for realizing high capacity, a technique of disposing a larger amount of data tracks in a width direction of the magnetic tape by narrowing the width of the data track to increase recording density is used.

However, in a case where the width of the data track is narrowed and the recording and/or reproducing of information is performed by allowing the running of the magnetic tape in a magnetic tape apparatus (normally referred to as a “drive”), it is difficult that a magnetic head correctly follows the data tracks due to the position change of the magnetic tape, and errors may easily occur at the time of recording and/or reproducing. Thus, as means for preventing occurrence of such errors, a system using a head tracking servo using a servo signal (hereinafter, referred to as a “servo system”) has been recently proposed and practically used (for example, see U.S. Pat. No. 5,689,384A).

SUMMARY OF THE INVENTION

In a magnetic servo type servo system among the servo systems, a servo pattern (servo signal) is formed in a magnetic layer of a magnetic tape, and this servo pattern is magnetically read to perform head tracking. More specific description is as follows.

First, a servo head reads a servo pattern formed in a magnetic layer (that is, reproduces a servo signal). A position of a magnetic head in a magnetic tape apparatus is controlled in accordance with a value obtained by reading the servo pattern. Accordingly, in a case of transporting the magnetic tape in the magnetic tape apparatus for recording and/or reproducing information, it is possible to increase an accuracy of the magnetic head following the data track, even in a case where the position of the magnetic tape is changed. For example, even in a case where the position of the magnetic tape is changed in the width direction with respect to the magnetic head, in a case of recording and/or reproducing information by transporting the magnetic tape in the magnetic tape apparatus, it is possible to control the position of the magnetic head in the width direction of the magnetic tape in the magnetic tape apparatus, by performing the head tracking servo. By doing so, it is possible to correctly record information on the magnetic tape and/or correctly reproduce information recorded on the magnetic tape in the magnetic tape apparatus.

By the way, a magnetic tape is generally accommodated in a magnetic tape cartridge, circulated, and used. In order to increase recording capacity for one reel of the magnetic tape cartridge, it is desirable to increase a total length of the magnetic tape accommodated in one reel of the magnetic tape cartridge. In order to increase the total length of the magnetic tape, it is necessary that a total thickness of the magnetic tape is thinned (hereinafter, also referred to as “thinning”).

In recent years, the magnetic tape is required to increase the surface smoothness of the magnetic layer. Increasing the surface smoothness of the magnetic layer leads to the improvement of electromagnetic conversion characteristics.

In view of the point described above, the present inventor studied application of the magnetic tape with the thinned total thickness and the increased surface smoothness of the magnetic layer to the servo system. However, in the studies, it was clear that, in a case where the total thickness of the magnetic tape is thinned and the surface smoothness of the magnetic layer is increased, a phenomenon which has not been known in the related art occurs in which an occurrence frequency of a signal defect increases at the time of reproducing the servo signal in the servo system. As an example of such a signal defect, a signal defect called thermal asperity is used. The thermal asperity is fluctuation in a reproduced waveform caused by change of a resistance value of a magnetoresistive (MR) element due to occurrence of a local temperature change in the MR element in a system comprising an MR head on which the MR element is mounted. In a case where the signal defect occurs at the time of reproducing the servo signal, it is difficult to perform head tracking at the occurrence location. Accordingly, it is required to reduce the occurrence frequency of the signal defect at the time of reproducing the servo signal, in order to more correctly record information on the magnetic tape and/or more correctly reproduce information recorded on the magnetic tape by using the servo system.

According to an aspect of the invention, an object is to reduce an occurrence frequency of a signal defect in a servo system in a magnetic tape with a thinned total thickness and an increased surface smoothness of a magnetic layer.

According to an aspect of the invention, there is provided a magnetic tape comprising: a non-magnetic support; and a magnetic layer including ferromagnetic powder and a binding agent on the non-magnetic support, in which a total thickness of the magnetic tape is equal to or smaller than 5.30 the magnetic layer has a servo pattern, a center line average surface roughness Ra measured on a surface of the magnetic layer (hereinafter, also referred to as a “magnetic layer surface roughness Ra”) is equal to or smaller than 1.8 nm, and an absolute value ΔN (hereinafter, also referred to as “ΔN (of the magnetic layer)”) of a difference between a refractive index Nxy of the magnetic layer, measured in an in-plane direction and a refractive index Nz of the magnetic layer, measured in a thickness direction is 0.25 to 0.40.

In an aspect, Nxy>Nz may be satisfied and the difference (Nxy−Nz) between the refractive index Nxy and the refractive index Nz may be 0.25 to 0.40.

In an aspect, the magnetic tape may further comprise a non-magnetic layer including non-magnetic powder and a binding agent between the non-magnetic support and the magnetic layer.

In an aspect, the total thickness of the magnetic tape may be 3.00 μm to 5.30 μm.

In an aspect, the magnetic tape may further comprise a back coating layer including non-magnetic powder and a binding agent on a surface of the non-magnetic support opposite to a surface provided with the magnetic layer.

In an aspect, the magnetic layer surface roughness Ra may be 1.2 nm to 1.8 nm.

In an aspect, the servo pattern may be a timing-based servo pattern.

According to another aspect of the invention, there is provided a magnetic tape cartridge comprising: the magnetic tape described above.

According to still another aspect of the invention, there is provided a magnetic tape apparatus comprising: the magnetic tape described above; and a magnetic head.

According to an aspect of the invention, it is possible to provide a magnetic tape which is thinned, has a servo pattern in a magnetic layer with high surface smoothness, and has a reduced occurrence frequency of a signal defect at the time of reproducing a servo signal in a servo system, and a magnetic tape cartridge and a magnetic tape apparatus including the magnetic tape.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of disposition of data bands and servo bands.

FIG. 2 shows a servo pattern disposition example of a linear-tape-open (LTO) Ultrium format tape.

FIG. 3 is a schematic illustration of an embodiment of a magnetic tape of the invention. The magnetic tape has a non-magnetic support, a magnetic layer including ferromagnetic powder and a binding agent on the non-magnetic support, a non-magnetic layer including non-magnetic powder and a binding agent between the non-magnetic support and the magnetic layer, and a back coating layer including non-magnetic powder and a binding agent on the surface of the non-magnetic support opposite to the surface provided with the magnetic layer.

FIG. 4 illustrates a generalized depiction of how the refractive index Nxy of the magnetic layer, measured in an in-plane direction, and the refractive index Nz of the magnetic layer, measured in a thickness direction, are measured. The incidence rays and the Nx, Ny and Nz directions are shown relative to the magnetic tape.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Magnetic Tape

An aspect of the invention relates to a magnetic tape including a non-magnetic support; and a magnetic layer including ferromagnetic powder and a binding agent on the non-magnetic support, in which a total thickness of the magnetic tape is equal to or smaller than 5.30 μm, the magnetic layer has a servo pattern, a magnetic layer surface roughness Ra is equal to or smaller than 1.8 nm, and ΔN of the magnetic layer is 0.25 to 0.40.

Hereinafter, the magnetic tape will be described more specifically. The following description includes a surmise of the inventor. The invention is not limited to such a surmise. In addition, hereinafter, exemplary description may be made with reference to the drawings. However, the invention is not limited to the exemplified aspects.

Magnetic Layer

Magnetic Layer Surface Roughness Ra

A center line average surface roughness Ra (magnetic layer surface roughness Ra) measured on a surface of the magnetic layer of the magnetic tape is equal to or smaller than 1.8 nm. In the magnetic tape having the magnetic layer surface roughness Ra of equal to or smaller than 1.8 nm and the total thickness of equal to or smaller than 5.30 μm, in a case where no measures are taken, an occurrence frequency of a signal defect increases at the time of reproducing a servo signal in a servo system. With respect to this, in the magnetic tape having ΔN of the magnetic layer of 0.25 to 0.40, it is possible to prevent occurrence of the signal defect at the time of reproducing the servo signal in spite of the magnetic layer surface roughness Ra of equal to or smaller than 1.8 nm and the total thickness of equal to or smaller than 5.30 μm. The surmise of the inventor regarding this point will be described later. In addition, the magnetic tape having the magnetic layer surface roughness Ra of equal to or smaller than 1.8 nm can exhibit excellent electromagnetic conversion characteristics. From a viewpoint of further improving the electromagnetic conversion characteristics, the magnetic layer surface roughness Ra is preferably equal to or smaller than 1.7 nm and more preferably equal to or smaller than 1.6 nm. In addition, the magnetic layer surface roughness Ra can be, for example, equal to or greater than 1.2 nm or equal to or greater than 1.3 nm. However, from the viewpoint of improving the electromagnetic conversion characteristics, since the value of the magnetic layer surface roughness Ra is preferably as small as possible, it may be less than the value exemplified above. In the invention and the specification, the “surface (of) the magnetic layer” of the magnetic tape has the same meaning as the surface of the magnetic tape on the magnetic layer side.

The center line average surface roughness Ra measured on the surface of the magnetic layer of the magnetic tape in the invention and the specification is a value measured in an area of 40 μm×40 μm of the surface of the magnetic layer by an atomic force microscope (AFM). As an example of measurement conditions, the following measurement conditions can be used. The magnetic layer surface roughness Ra shown in examples which will be described later is a value measured under the following measurement conditions.

A region having the area of 40 μm×40 μm of the surface of the magnetic layer of the magnetic tape is measured by using the AFM (Nanoscope 4 manufactured by Veeco Instruments Inc.) in a tapping mode. RTESP-300 manufactured by Bruker Japan K.K. is used as a probe, a scan speed (probe moving speed) is 40 μm/sec, and a resolution is 512 pixels×512 pixels.

The magnetic layer surface roughness Ra can be controlled by a well-known method. For example, the magnetic layer surface roughness Ra can vary depending on sizes of various powder included in the magnetic layer (for example, ferromagnetic powder, non-magnetic powder which can be randomly obtained therein, or the like), manufacturing conditions of the magnetic tape, and the like. Accordingly, the magnetic tape having the magnetic layer surface roughness Ra of equal to or smaller than 1.8 nm can be obtained by adjusting these.

Servo Pattern

The magnetic tape includes a servo pattern in the magnetic layer. The formation of the servo pattern on the magnetic layer is performed by magnetizing a specific position of the magnetic layer by a servo write head. A shape of the servo pattern and disposition thereof in the magnetic layer for realizing the head tracking servo are well known. In regards to the servo pattern of the magnetic layer of the magnetic tape, a well-known technique can be used. For example, as a head tracking servo system, a timing-based servo system and an amplitude-based servo system are known. The servo pattern of the magnetic layer of the magnetic tape may be a servo pattern capable of allowing head tracking servo of any system. In addition, a servo pattern capable of allowing head tracking servo in the timing-based servo system and a servo pattern capable of allowing head tracking servo in the amplitude-based servo system may be formed in the magnetic layer.

Hereinafter, as one specific aspect of the head tracking servo, head tracking servo in the timing-based servo system will be described. However, the head tracking servo in the timing-based servo system of the invention is not limited to the following specific aspect.

In the head tracking servo in the timing-based servo system (hereinafter, referred to as “timing-based servo”), a plurality of servo patterns having two or more different shapes are formed on a magnetic layer, and a position of a servo head is recognized by an interval of time in a case where the servo head has read two servo patterns having different shapes and an interval of time in a case where the servo head has read two servo patterns having the same shapes. The position of the magnetic head in the width direction of the magnetic tape is controlled based on the position of the servo head recognized as described above. In an aspect, the magnetic head, the position of which is controlled here, is a magnetic head (reproducing head) which reproduces information recorded on the magnetic tape, and in another aspect, the magnetic head is a magnetic head (recording head) which records information in the magnetic tape.

FIG. 1 shows an example of disposition of data bands and servo bands. In FIG. 1, a plurality of servo bands 10 are disposed to be interposed between guide bands 12 in a magnetic layer of a magnetic tape 1. A plurality of regions 11 each of which is interposed between two servo bands are data bands. The servo pattern is a magnetized region and is formed by magnetizing a specific region of the magnetic layer by a servo write head. The region magnetized by the servo write head (position where a servo pattern is formed) is determined by standards. For example, in an LTO Ultrium format tape which is based on a local standard, a plurality of servo patterns tilted in a tape width direction as shown in FIG. 2 are formed on a servo band, in a case of manufacturing a magnetic tape. Specifically, in FIG. 2, a servo frame SF on the servo band 10 is configured with a servo sub-frame 1 (SSF1) and a servo sub-frame 2 (SSF2). The servo sub-frame 1 is configured with an A burst (in FIG. 2, reference numeral A) and a B burst (in FIG. 2, reference numeral B). The A burst is configured with servo patterns A1 to A5 and the B burst is configured with servo patterns B1 to B5. Meanwhile, the servo sub-frame 2 is configured with a C burst (in FIG. 2, reference numeral C) and a D burst (in FIG. 2, reference numeral D). The C burst is configured with servo patterns Cl to C4 and the D burst is configured with servo patterns D1 to D4. Such 18 servo patterns are disposed in the sub-frames in the arrangement of 5, 5, 4, 4, as the sets of 5 servo patterns and 4 servo patterns, and are used for recognizing the servo frames. FIG. 2 shows one servo frame for explaining. However, in practice, in the magnetic layer of the magnetic tape in which the head tracking servo in the timing-based servo system is performed, a plurality of servo frames are disposed in each servo band in a running direction. In FIG. 2, an arrow shows the running direction. For example, an LTO Ultrium format tape generally includes 5,000 or more servo frames per a tape length of 1 m, in each servo band of the magnetic layer. The servo head sequentially reads the servo patterns in the plurality of servo frames, while coming into contact with and sliding on the surface of the magnetic layer of the magnetic tape transported in the magnetic tape apparatus.

In the head tracking servo in the timing-based servo system, a position of a servo head is recognized based on an interval of time in a case where the servo head has read the two servo patterns (reproduced servo signals) having different shapes and an interval of time in a case where the servo head has read two servo patterns having the same shapes. The interval of time is normally obtained as an interval of time of a peak of a reproduced waveform of a servo signal. For example, in the aspect shown in FIG. 2, the servo pattern of the A burst and the servo pattern of the C burst are servo patterns having the same shapes, and the servo pattern of the B burst and the servo pattern of the D burst are servo patterns having the same shapes. The servo pattern of the A burst and the servo pattern of the C burst are servo patterns having the shapes different from the shapes of the servo pattern of the B burst and the servo pattern of the D burst. An interval of the time in a case where the two servo patterns having different shapes are read by the servo head is, for example, an interval between the time in a case where any servo pattern of the A burst is read and the time in a case where any servo pattern of the B burst is read. An interval of the time in a case where the two servo patterns having the same shapes are read by the servo head is, for example, an interval between the time in a case where any servo pattern of the A burst is read and the time in a case where any servo pattern of the C burst is read. The head tracking servo in the timing-based servo system is a system supposing that occurrence of a deviation of the interval of time is due to a position change of the magnetic tape in the width direction, in a case where the interval of time is deviated from the set value. The set value is an interval of time in a case where the magnetic tape runs without occurring the position change in the width direction. In the timing-based servo system, the magnetic head is moved in the width direction in accordance with a degree of the deviation of the obtained interval of time from the set value. Specifically, as the interval of time is greatly deviated from the set value, the magnetic head is greatly moved in the width direction. This point is applied to not only the aspect shown in FIGS. 1 and 2, but also to entire timing-based servo systems.

For example, in a magnetic tape apparatus using the timing-based servo system, in a case where the signal defect occurs at the time of reproducing the servo signal, it is difficult to obtain measurement results of the interval of time at a location (servo frame) where the defect occurs. As a result, it is partially difficult to position the head by moving the magnetic head in the width direction in a case of recording or reproducing a magnetic signal (information) by a magnetic head by allowing the magnetic tape to run. The occurrence of the signal defect at the time of reproducing the servo signal in the magnetic tape apparatus using the servo system as well as the timing-based servo system makes it difficult to position the head by moving the magnetic head in a case of recording or reproducing a magnetic signal (information) by the magnetic head by allowing the magnetic tape to run.

In regards to the point described above, in the studies of the inventor, it was found that, in the magnetic tape having the total thickness of equal to or smaller than 5.30 μm and the magnetic layer surface roughness Ra of equal to or smaller than 1.8 nm, the signal defect significantly occurs at the time of reproducing the servo signal. The inventor has considered that a reason of the occurrence of the signal defect at the time of reproducing the servo signal is that a smooth sliding between the servo head and the surface of the magnetic layer is hindered (hereinafter, referred to as a “decrease in sliding”). The inventor has surmised that a reason of the decrease in sliding is that the magnetic tape having the total thickness of equal to or smaller than 5.30 μm and the magnetic layer surface roughness Ra of equal to or smaller than 1.8 nm is different from the magnetic tape of the related art in a contact state between the servo head and the surface of the magnetic layer. However, it is merely a surmise.

As a result of the intensive studies of the inventor with respect to this, it was clear that such occurrence of the signal defect at the time of reproducing the servo signal can be prevented by setting ΔN of the magnetic layer to be 0.25 to 0.40. The surmise of the inventor regarding this point will be described later.

ΔN of Magnetic Layer

In the invention and the specification, the absolute value ΔN of the difference between the refractive index Nxy of the magnetic layer, measured in an in-plane direction and the refractive index Nz of the magnetic layer, measured in a thickness direction is a value obtained by the following method.

A refractive index of the magnetic layer in each direction is obtained by a spectral ellipsometry using a double-layer model. In order to obtain a refractive index of the magnetic layer by a spectral ellipsometry using a double-layer model, a value of a refractive index of a portion adjacent to the magnetic layer is used. Hereinafter, an example in a case of obtaining the refractive indexes Nxy and Nz of the magnetic layer in a magnetic tape having a layer configuration in which a non-magnetic layer and a magnetic layer are laminated on a non-magnetic support in this order will be described. However, the magnetic tape according to an aspect of the invention may also be a magnetic tape having a layer configuration in which a magnetic layer is directly laminated on a non-magnetic support without the non-magnetic layer interposed therebetween. Regarding the magnetic tape having such a configuration, a refractive index of the magnetic layer in each direction is obtained in the same manner as the following method, using a double-layer model of a magnetic layer and a non-magnetic support. Moreover, an incidence angle shown below is an incidence angle in a case where an incidence angle is 0° in a case of normal incidence.

(1) Preparation of Sample for Measurement

Regarding a magnetic tape including a back coating layer on a surface of a non-magnetic support on a side opposite to the surface provided with a magnetic layer, the measurement is performed after removing the back coating layer of a sample for measurement cut from the magnetic tape. The removal of the back coating layer can be performed by a well-known method of dissolving of the back coating layer using a solvent or the like. As the solvent, for example, methyl ethyl ketone can be used. However, any solvent which can remove the back coating layer may be used. The surface of the non-magnetic support after removing the back coating layer is roughened by a well-known method so that the reflected light on this surface is not detected, in the measurement of ellipsometer. The roughening can be performed by a well-known method such as polishing the surface of the non-magnetic support after removing the back coating layer by using sand paper, for example. Regarding the sample for measurement cut out from the magnetic tape not including the back coating layer, the surface of the non-magnetic support on a side opposite to the surface provided with the magnetic layer is roughened.

In addition, in order to measure the refractive index of the non-magnetic layer described below, the magnetic layer is further removed and the surface of the non-magnetic layer is exposed. In order to measure the refractive index of the non-magnetic support described below, the non-magnetic layer is also further removed and the surface of the non-magnetic support on the magnetic layer side is exposed. The removal of each layer can be performed by a well-known method so as described regarding the removal of the back coating layer. A longitudinal direction described below is a direction which was a longitudinal direction of the magnetic tape, in a case where the sample for measurement is included in the magnetic tape before being cut out. This point applies to other directions described below, in the same manner.

(2) Measurement of Refractive Index of Magnetic Layer

Δ (phase difference of s-polarized light and p-polarized light) and Ψ (amplitude ratio of s-polarized light and p-polarized light) are measured, using an ellipsometer, by setting an incidence angle to 65°, 70°, and 75° and irradiating a magnetic layer surface with an incidence ray having a beam diameter of 300 μm in a longitudinal direction. The measurement is performed by changing a wavelength of an incidence ray by every 1.5 nm in a range of 400 to 700 nm, and a measurement value at each wavelength is obtained.

The refractive index of the magnetic layer at each wavelength is obtained with a double-layer model as described below, by using the measurement values of Δ and Ψ of the magnetic layer at each wavelength, the refractive index of the non-magnetic layer in each direction obtained by the following method, and the thickness of the magnetic layer.

The zeroth layer which is a substrate of the double-layer model is set as a non-magnetic layer and the first layer thereof is set as a magnetic layer. The double-layer model is created by assuming that there is no effect of rear surface reflection of the non-magnetic layer, by only considering the reflection of the interfaces of air/magnetic layer and magnetic layer/non-magnetic layer. A refractive index of the first layer that most closely matches the obtained measurement value is obtained by fitting the measurement value by a least squares method. A refractive index Nx of the magnetic layer in a longitudinal direction and a refractive index Nz₁ of the magnetic layer in a thickness direction measured by emitting an incidence ray in a longitudinal direction are obtained as values at the wavelength of 600 nm obtained from the results of the fitting.

In the same manner as described above, except that a direction in which an incidence ray is incident is set as a width direction of the magnetic tape, a refractive index Ny of the magnetic layer in a width direction and a refractive index Nz₂ of the magnetic layer in a thickness direction measured by emitting an incidence ray in a width direction are obtained as values at the wavelength of 600 nm obtained from the results of the fitting.

Fitting is performed by the following method.

In general, “complex refractive index n=η+ix”. Here, η is a real number of the refractive index, κ is an extinction coefficient, and i is an imaginary number. In a case where a complex dielectric constant ε=ε1+1ε2 (ε1 and ε2 satisfies Kramers-Kronig relation), ε1=η²−κ², and ε2=2ηκ, the complex dielectric constant of Nx satisfies that ε_(x)=ε_(x)1+iε_(x)2, and the complex dielectric constant of Nz₁ satisfies that ε_(z1)1=ε_(z1)1+iε_(z1)2, in a case of calculating the Nx and Nz₁.

The Nx is obtained by setting ε_(x)2 as one Gaussian, setting any point, where a peak position is 5.8 to 5.1 eV and σ is 4 to 3.5 eV, as a starting point, setting a parameter to be offset to a dielectric constant beyond a measurement wavelength range (400 to 700 nm), and performing least squares fitting with respect to the measurement value. In the same manner, Nz₁ is obtained by setting any point of ε_(z1)2, where a peak position is 3.2 to 2.9 eV and σ is 1.5 to 1.2 eV, as a starting point, and setting an offset parameter, and performing least squares fitting with respect to the measurement value. Ny and Nz₂ are also obtained in the same manner. The refractive index Nxy, measured in an in-plane direction of the magnetic layer is obtained as “Nxy=(Nx+Ny)/2”. The refractive index Nz of the magnetic layer, measured in a thickness direction is obtained as “Nz=(Nz₁+Nz₂)/2”. From the obtained Nxy and Nz, the absolute value ΔN of difference thereof is obtained.

(3) Measurement of Refractive Index of Non-Magnetic Layer

Refractive indexes of the non-magnetic layer at a wavelength of 600 nm (the refractive index in the longitudinal direction, the refractive index in the width direction, the refractive index in a thickness direction measured by emitting an incidence ray in a longitudinal direction, and the refractive index in a thickness direction measured by emitting an incidence ray in a width direction) are obtained in the same manner as in the method described above, except the following points.

A wavelength of an incidence ray is changed by every 1.5 nm in the range of 250 to 700 nm.

By using a double-layer model of a non-magnetic layer and a non-magnetic support, the zeroth layer which is a substrate of the double-layer model is set as the non-magnetic support, and the first layer thereof is set as the non-magnetic layer. The double-layer model is created by assuming that there is no effect of rear surface reflection of the non-magnetic support, by only considering the reflection of the interfaces of air/non-magnetic layer and non-magnetic layer/non-magnetic support.

In the fitting, seven peaks (0.6 eV, 2.3 eV, 2.9 eV, 3.6 eV, 4.6 eV, 5.0 eV, and 6.0 eV) are assumed in the imaginary part (ε2) of the complex dielectric constant, and the parameter to be offset is set to the dielectric constant beyond the measurement wavelength range (250 to 700 nm).

(4) Measurement of Refractive Index of Non-Magnetic Support

The refractive indexes of the non-magnetic support at a wavelength of 600 nm (refractive index in the longitudinal direction, the refractive index in the width direction, the refractive index in a thickness direction measured by emitting an incidence ray in a longitudinal direction, and the refractive index in a thickness direction measured by emitting an incidence ray in a width direction) used for obtaining the refractive indexes of the non-magnetic layer by the double-layer model are obtained in the same manner as in the method described above for measuring the refractive index of the magnetic layer, except the following points.

A single-layer model with only front surface reflection is used, without using the double-layer model.

Fitting is performed by a Cauchy model (n=A+B/λ², n is a refractive index, A and B are respectively constants determined by fitting, and λ is a wavelength).

The occurrence of the signal defect at the time of reproducing the servo signal in the magnetic tape having the total thickness of equal to or smaller than 5.30 μm and the magnetic layer surface roughness Ra of equal to or smaller than 1.8 nm can be prevented by setting ΔN of the magnetic layer obtained by the method described above to be 0.25 to 0.40. The occurrence of the signal defect is considered to be caused by the decrease in sliding between the servo head and the surface of the magnetic layer. On the other hand, the inventor has considered that ΔN obtained by the method described above is a value which may be an index of a presence state of the ferromagnetic powder in a surface region of the magnetic layer. This ΔN is surmised as a value which is influenced by the effect of various factors such as a presence state of a binding agent or a density distribution of the ferromagnetic powder, in addition to the alignment state of the ferromagnetic powder in the magnetic layer. In addition, it is considered that the magnetic layer in which the ΔN is set as 0.25 to 0.40 by controlling various factors has a high hardness of the surface of the magnetic layer and the chipping thereof hardly occurs even under the sliding with the servo head. As a result, the inventor has surmised that the prevention of the attachment of the scraps generated due to the chipping of the surface of the magnetic layer as a foreign material onto the servo head contributes to the prevention of the decrease in sliding between the servo head and the surface of the magnetic layer. The inventor has considered that this leads to the prevention of the occurrence of the signal defect. However, this is merely a surmise and the invention is not limited to the surmise.

ΔN of the magnetic layer of the magnetic tape is 0.25 to 0.40. From a viewpoint of further preventing the occurrence of the signal defect, ΔN is preferably 0.25 to 0.35. A specific aspect of means for adjusting ΔN will be described later.

ΔN is an absolute value of a difference between Nxy and Nz. Nxy is a refractive index of the magnetic layer, measured in an in-plane direction and Nz is a refractive index of the magnetic layer, measured in a thickness direction. In an aspect, a relation of Nxy>Nz can be satisfied, and in the other aspect, Nxy<Nz can be satisfied. From a viewpoint of electromagnetic conversion characteristics of the magnetic tape, a relationship of Nxy>Nz is preferable, and therefore, the difference between the Nxy and Nz (Nxy−Nz) is preferably 0.25 to 0.40 and more preferably 0.25 to 0.35. In an aspect, Nxy can be, for example, 1.50 to 2.50. In an aspect, Nz can be, for example, 1.30 to 2.50. However, the magnetic tape may have ΔN of 0.25 to 0.40, and Nxy and Nz are not limited to the ranges exemplified above.

Various means for adjusting ΔN described above will be described later.

Next, the magnetic layer will be described in detail.

Ferromagnetic Powder

As the ferromagnetic powder included in the magnetic layer, ferromagnetic powder normally used in the magnetic layer of various magnetic recording media can be used. It is preferable to use ferromagnetic powder having a small average particle size, from a viewpoint of improvement of recording density of the magnetic recording medium. From this viewpoint, ferromagnetic powder having an average particle size equal to or smaller than 50 nm is preferably used, and ferromagnetic powder having an average particle size equal to or smaller than 40 nm is more preferably used, as the ferromagnetic powder. On the other hand, from a viewpoint of stability of magnetization, the average particle size of the ferromagnetic powder is preferably equal to or greater than 5 nm, more preferably equal to or greater than 10 nm, and even more preferably equal to or greater than 15 nm.

As a preferred specific example of the ferromagnetic powder, hexagonal ferrite powder can be used. The hexagonal ferrite powder can be barium ferrite, strontium ferrite, calcium ferrite, lead ferrite, or the like, or may be a mixed crystal of two or more kinds of these. For details of the hexagonal ferrite powder, descriptions disclosed in paragraphs 0012 to 0030 of JP2011-225417A, paragraphs 0134 to 0136 of JP2011-216149A, paragraphs 0013 to 0030 of JP2012-204726A, and paragraphs 0029 to 0084 of JP2015-127985A can be referred to, for example.

As a preferred specific example of the ferromagnetic powder, metal powder can also be used. For details of the metal powder, descriptions disclosed in paragraphs 0137 to 0141 of JP2011-216149A and paragraphs 0009 to 0023 of JP2005-251351A can be referred to, for example.

As a preferred specific example of the ferromagnetic powder, ε-iron oxide powder can also be used. As a manufacturing method of the s-iron oxide powder, a manufacturing method from a goethite, a reverse micelle method, and the like are known. All of the manufacturing methods are well known. Regarding a method of manufacturing the ε-iron oxide powder in which a part of Fe is substituted with substitutional atoms such as Ga, Co, Ti, Al, or Rh, a description disclosed in J. Jpn. Soc. Powder Metallurgy Vol. 61 Supplement, No. 51, pp. S280 to S284, J. Mater. Chem. C, 2013, 1, pp. 5200 to 5206 can be referred, for example. However, the manufacturing method of the ε-iron oxide powder capable of being used as the ferromagnetic powder in the magnetic layer is not limited.

In the invention and the specification, “ferromagnetic powder” means an aggregate of a plurality of ferromagnetic particles. The “aggregation” includes not only an aspect in which the particles constituting the aggregation are in direct contact with each other, but also an aspect in which a binding agent, an additive, and the like are interposed between the particles is also included. The point described above applies to various powder such as the non-magnetic powder in the invention and the specification in the same manner.

In the invention and the specification, average particle sizes of various powder such as the ferromagnetic powder are values measured by the following method with a transmission electron microscope, unless otherwise noted.

The powder is imaged at a magnification ratio of 100,000 with a transmission electron microscope, the image is printed on photographic printing paper so as to have the total magnification ratio of 500,000 to obtain an image of particles constituting the powder. A target particle is selected from the obtained image of particles, an outline of the particle is traced with a digitizer, and a size of the particle (primary particle) is measured. The primary particle is an independent particle which is not aggregated.

The measurement described above is performed regarding 500 particles randomly extracted. An arithmetic mean of the particle size of 500 particles obtained as described above is an average particle size of the powder. As the transmission electron microscope, a transmission electron microscope H-9000 manufactured by Hitachi, Ltd. can be used, for example. In addition, the measurement of the particle size can be performed by well-known image analysis software, for example, image analysis software KS-400 manufactured by Carl Zeiss. The value regarding the size of the powder such as the average particle size shown in examples which will be described later is a value measured by using transmission electron microscope H-9000 manufactured by Hitachi, Ltd. as the transmission electron microscope, and image analysis software KS-400 manufactured by Carl Zeiss as the image analysis software, unless otherwise noted.

As a method of collecting sample powder from the magnetic tape in order to measure the particle size, a method disclosed in a paragraph 0015 of JP2011-048878A can be used, for example.

In the invention and the specification, unless otherwise noted, (1) in a case where the shape of the particle observed in the particle image described above is a needle shape, a fusiform shape, or a columnar shape (here, a height is greater than a maximum long diameter of a bottom surface), the size (particle size) of the particles constituting the powder is shown as a length of a long axis configuring the particle, that is, a long axis length, (2) in a case where the shape of the particle is a planar shape or a columnar shape (here, a thickness or a height is smaller than a maximum long diameter of a plate surface or a bottom surface), the particle size is shown as a maximum long diameter of the plate surface or the bottom surface, and (3) in a case where the shape of the particle is a sphere shape, a polyhedron shape, or an unspecified shape, and the long axis configuring the particles cannot be specified from the shape, the particle size is shown as an equivalent circle diameter. The equivalent circle diameter is a value obtained by a circle projection method.

In addition, regarding an average acicular ratio of the powder, a length of a short axis, that is, a short axis length of the particles is measured in the measurement described above, a value of (long axis length/short axis length) of each particle is obtained, and an arithmetic mean of the values obtained regarding 500 particles is calculated. Here, unless otherwise noted, in a case of (1), the short axis length as the definition of the particle size is a length of a short axis configuring the particle, in a case of (2), the short axis length is a thickness or a height, and in a case of (3), the long axis and the short axis are not distinguished, thus, the value of (long axis length/short axis length) is assumed as 1, for convenience.

In addition, unless otherwise noted, in a case where the shape of the particle is specified, for example, in a case of definition of the particle size (1), the average particle size is an average long axis length, in a case of the definition (2), the average particle size is an average plate diameter. In a case of the definition (3), the average particle size is an average diameter (also referred to as an average particle diameter).

In an aspect, the shape of the ferromagnetic particles constituting the ferromagnetic powder included in the magnetic layer can be a plate shape. Hereinafter, the ferromagnetic powder including the plate-shaped ferromagnetic particles is referred to as a plate-shaped ferromagnetic powder. An average plate ratio of the plate-shaped ferromagnetic powder can be preferably 2.5 to 5.0. The average plate ratio is an arithmetic mean of (maximum long diameter/thickness or height) in a case of the definition (2). As the average plate ratio increases, uniformity of the alignment state of the ferromagnetic particles constituting the plate-shaped ferromagnetic powder tends to easily increase by the alignment process, and the value of ΔN tends to increase.

As an index for a particle size of the ferromagnetic powder, an activation volume can be used. The “activation volume” is a unit of magnetization reversal. Regarding the activation volume described in the invention and the specification, magnetic field sweep rates of a coercivity Hc measurement part at time points of 3 minutes and 30 minutes are measured by using a vibrating sample magnetometer in an environment of an atmosphere temperature 23° C.±1° C., and the activation volume is a value acquired from the following relational expression of Hc and an activation volume V. The activation volume shown in examples which will be described later is a value acquired by performing the measurement using a vibrating sample magnetometer manufactured by Toei Industry Co., Ltd. Hc=2Ku/Ms{1−[(kT/KuV)ln(At/0.693)]^(1/2)}

[In the expression, Ku: anisotropy constant, Ms: saturation magnetization, k: Boltzmann's constant, T: absolute temperature, V: activation volume, A: spin precession frequency, and t: magnetic field reversal time]

From a viewpoint of improvement of recording density, the activation volume of the ferromagnetic powder is preferably equal to or smaller than 2,500 nm³, more preferably equal to or smaller than 2,300 nm³, and even more preferably equal to or smaller than 2,000 nm³. On the other hand, from a viewpoint of stability of magnetization, the activation volume of the ferromagnetic powder is, for example, preferably equal to or greater than 800 nm³, more preferably equal to or greater than 1,000 nm³, and even more preferably equal to or greater than 1,200 nm³.

The content (filling percentage) of the ferromagnetic powder of the magnetic layer is preferably 50% to 90% by mass and more preferably 60% to 90% by mass. The components other than the ferromagnetic powder of the magnetic layer are at least a binding agent, and one or more kinds of additives may be randomly included. A high filling percentage of the ferromagnetic powder in the magnetic layer is preferable from a viewpoint of improvement of recording density.

Binding Agent and Curing Agent

The magnetic tape is a coating type magnetic tape and includes a binding agent in the magnetic layer. The binding agent is one or more kinds of resin. The resin may be a homopolymer or a copolymer. As the binding agent included in the magnetic layer, a resin selected from a polyurethane resin, a polyester resin, a polyamide resin, a vinyl chloride resin, an acrylic resin obtained by copolymerizing styrene, acrylonitrile, or methyl methacrylate, a cellulose resin such as nitrocellulose, an epoxy resin, a phenoxy resin, and a polyvinylalkylal resin such as polyvinyl acetal or polyvinyl butyral can be used alone or a plurality of resins can be mixed with each other to be used. Among these, a polyurethane resin, an acrylic resin, a cellulose resin, and a vinyl chloride resin are preferable. These resins can be used as the binding agent even in the non-magnetic layer and/or a back coating layer which will be described later. For the binding agent described above, description disclosed in paragraphs 0029 to 0031 of JP2010-024113A can be referred to. In addition, the binding agent may be a radiation curable resin such as an electron beam curable resin. For the radiation curable resin, a description disclosed in paragraphs 0044 and 0045 of JP2011-048878A can be referred to.

An average molecular weight of the resin used as the binding agent can be, for example, 10,000 to 200,000 as a weight-average molecular weight. The weight-average molecular weight of the invention and the specification is a value obtained by performing polystyrene conversion of a value measured by gel permeation chromatography (GPC). As measurement conditions, the following conditions can be used. The weight-average molecular weight shown in examples which will be described later is a value obtained by performing polystyrene conversion of a value measured under the following measurement conditions.

GPC device: HLC-8120 (manufactured by Tosoh Corporation)

Column: TSK gel Multipore HXL-M (manufactured by Tosoh Corporation, 7.8 mmID (inner diameter)×30.0 cm)

Eluent: Tetrahydrofuran (THF)

In an aspect, as the binding agent, a binding agent including an acidic group can be used. The acidic group of the invention and the specification is used as a meaning including a form of a group capable of emitting H⁺ in water or a solvent including water (aqueous solvent) to be dissociated into anions and a salt thereof. As specific examples of the acidic group, a form of a sulfonic acid group, a sulfuric acid group, a carboxy group, a phosphoric acid group, and a salt thereof can be used. For example, a form of a salt of a sulfonic acid group (—SO₃H) means a group represented by —SO₃M, where M represents a group representing an atom (for example, alkali metal atom or the like) which may be cations in water or in an aqueous solvent. The same applies to the form of each of salts of the various groups described above. As an example of the binding agent including the acidic group, a resin including at least one kind of acidic group selected from the group consisting of a sulfonic acid group and a salt thereof (for example, a polyurethane resin or a vinyl chloride resin) can be used. However, the resin included in the magnetic layer is not limited to these resins. In addition, in the binding agent including the acidic group, a content of the acidic group can be, for example, 20 to 500 eq/ton. The unit “eq” represents equivalent, and is a unit that cannot be converted into an SI unit. The content of various functional groups such as the acidic group included in the resin can be obtained by a well-known method in accordance with the kind of the functional group. As the binding agent having a great content of the acidic group is used, the value of ΔN tends to increase. The amount of the binding agent used in a magnetic layer forming composition can be, for example, 1.0 to 30.0 parts by mass, and preferably 1.0 to 20.0 parts by mass with respect to 100.0 parts by mass of the ferromagnetic powder. As the amount of the binding agent used with respect to the ferromagnetic powder increases, the value of ΔN tends to increase.

In addition, a curing agent can also be used together with the resin which can be used as the binding agent. As the curing agent, in an aspect, a thermosetting compound which is a compound in which a curing reaction (crosslinking reaction) proceeds due to heating can be used, and in another aspect, a photocurable compound in which a curing reaction (crosslinking reaction) proceeds due to light irradiation can be used. At least a part of the curing agent is included in the magnetic layer in a state of being reacted (crosslinked) with other components such as the binding agent, by proceeding the curing reaction in the magnetic layer forming step. This point also applies to a layer formed by using a composition, in a case where the composition used for forming the other layer includes the curing agent. The preferred curing agent is a thermosetting compound, polyisocyanate being suitable. For details of the polyisocyanate, descriptions disclosed in paragraphs 0124 and 0125 of JP2011-216149A can be referred to. The amount of the curing agent can be, for example, 0 to 80.0 parts by mass with respect to 100.0 parts by mass of the binding agent in the magnetic layer forming composition, and is preferably 50.0 to 80.0 parts by mass, from a viewpoint of improvement of hardness of the magnetic layer.

Additives

The magnetic layer includes ferromagnetic powder and a binding agent, and may include one or more kinds of additives, if necessary. As the additives, the curing agent described above is used as an example. In addition, examples of the additive included in the magnetic layer include non-magnetic powder, a lubricant, a dispersing agent, a dispersing assistant, an antibacterial agent, an antistatic agent, an antioxidant, and carbon black. As the additives, a commercially available product can be suitably selected and used according to the desired properties. For example, for the lubricant, a description disclosed in paragraphs 0030 to 0033, 0035, and 0036 of JP2016-126817A can be referred to. The non-magnetic layer may include the lubricant. For the lubricant which may be included in the non-magnetic layer, a description disclosed in paragraphs 0030, 0031, 0034, 0035, and 0036 of JP2016-126817A can be referred to. For the dispersing agent, a description disclosed in paragraphs 0061 and 0071 of JP2012-133837A can be referred to. The dispersing agent may be included in the non-magnetic layer. For the dispersing agent which may be included in the non-magnetic layer, a description disclosed in a paragraph 0061 of JP2012-133837A can be referred to.

The magnetic layer preferably includes one kind or two or more kinds of non-magnetic powders. As the non-magnetic powder, non-magnetic powder (hereinafter, referred to as a “projection formation agent”) which can function as a projection formation agent which forms projections suitably protruded from the surface of the magnetic layer can be used. The projection formation agent is a component which can contribute to control of friction properties of the surface of the magnetic layer of the magnetic tape. In addition, the magnetic layer may include non-magnetic powder (hereinafter, referred to as an “abrasive”) which can function as an abrasive. The magnetic layer of the magnetic tape preferably includes at least one of the projection formation agent or the abrasive and more preferably includes both of them.

As the projection formation agent, various non-magnetic powders normally used as a projection formation agent can be used. These may be powder of an inorganic substance or powder of an organic substance. In an aspect, from a viewpoint of homogenization of friction properties, particle size distribution of the projection formation agent is not polydispersion having a plurality of peaks in the distribution and is preferably monodisperse showing a single peak. From a viewpoint of availability of monodisperse particles, the non-magnetic powder included in the magnetic layer is preferably powder of inorganic substances (inorganic powder). Examples of the inorganic powder include powder of inorganic oxide such as metal oxide, metal carbonate, metal sulfate, metal nitride, metal carbide, and metal sulfide, and powder of inorganic oxide is preferable. The projection formation agent is more preferably colloidal particles and even more preferably inorganic oxide colloidal particles. In addition, from a viewpoint of availability of monodisperse particles, the inorganic oxide colloidal particles are more preferably colloidal silica (silica colloidal particles). In the invention and the specification, the “colloidal particles” are particles which are not precipitated and dispersed to generate a colloidal dispersion, in a case where 1 g of the particles is added to 100 mL of at least one organic solvent of at least methyl ethyl ketone, cyclohexanone, toluene, or ethyl acetate, or a mixed solvent including two or more kinds of the solvent described above at any mixing ratio. The average particle size of the colloidal particles is a value obtained by a method disclosed in a paragraph 0015 of JP2011-048878A as a measurement method of an average particle diameter. In addition, in another aspect, the projection formation agent is preferably carbon black.

An average particle size of the projection formation agent is, for example, 30 to 300 nm and is preferably 40 to 200 nm.

Meanwhile, the abrasive is preferably non-magnetic powder having Mohs hardness exceeding 8 and more preferably non-magnetic powder having Mohs hardness equal to or greater than 9. A maximum value of Mohs hardness is 10 of diamond. Specifically, powders of alumina (Al₂O₃), silicon carbide, boron carbide (B₄C), SiO₂, TiC, chromium oxide (Cr₂O₃), cerium oxide, zirconium oxide (ZrO₂), iron oxide, diamond, and the like can be used, and among these, alumina powder such as α-alumina and silicon carbide powder are preferable. Regarding the particle size of the abrasive, a specific surface area which is an index of a particle size is, for example, equal to or greater than 14 m²/g, preferably equal to or greater than 16 m²/g, and more preferably equal to or greater than 18 m²/g. In addition, the specific surface area of the abrasive can be, for example, equal to or smaller than 40 m²/g. The specific surface area is a value obtained by a nitrogen adsorption method (also referred to as a Brunauer-Emmett-Teller (BET) one-point method). Hereinafter, the specific surface area obtained by such a method is also referred to as a BET specific surface area.

In addition, from a viewpoint that the projection formation agent and the abrasive can exhibit each function in more excellent manner, the content of the projection formation agent of the magnetic layer is preferably 1.0 to 4.0 parts by mass and more preferably 1.5 to 3.5 parts by mass with respect to 100.0 parts by mass of the ferromagnetic powder. Meanwhile, the content of the abrasive in the magnetic layer is preferably 1.0 to 20.0 parts by mass, more preferably 3.0 to 15.0 parts by mass, and even more preferably 4.0 to 10.0 parts by mass with respect to 100.0 parts by mass of the ferromagnetic powder.

As an example of the additive which can be used in the magnetic layer including the abrasive, a dispersing agent disclosed in paragraphs 0012 to 0022 of JP2013-131285A can be used as a dispersing agent for improving dispersibility of the abrasive of the magnetic layer forming composition.

Non-Magnetic Layer

Next, the non-magnetic layer will be described. The magnetic tape may include a magnetic layer directly on the non-magnetic support or may include a non-magnetic layer including the non-magnetic powder and the binding agent between the non-magnetic support and the magnetic layer. The non-magnetic powder used in the non-magnetic layer may be powder of inorganic substances or powder of organic substances. In addition, carbon black and the like can be used. Examples of the inorganic substance include metal, metal oxide, metal carbonate, metal sulfate, metal nitride, metal carbide, and metal sulfide. The non-magnetic powder can be purchased as a commercially available product or can be manufactured by a well-known method. For details thereof, descriptions disclosed in paragraphs 0146 to 0150 of JP2011-216149A can be referred to. For carbon black which can be used in the non-magnetic layer, descriptions disclosed in paragraphs 0040 and 0041 of JP2010-024113A can be referred to. The content (filling percentage) of the non-magnetic powder of the non-magnetic layer is preferably 50% to 90% by mass and more preferably 60% to 90% by mass.

In regards to other details of a binding agent or additives of the non-magnetic layer, the well-known technique regarding the non-magnetic layer can be applied. In addition, in regards to the type and the content of the binding agent, and the type and the content of the additive, for example, the well-known technique regarding the magnetic layer can be applied.

The non-magnetic layer of the magnetic tape also includes a substantially non-magnetic layer including a small amount of ferromagnetic powder as impurities or intentionally, together with the non-magnetic powder. Here, the substantially non-magnetic layer is a layer having a residual magnetic flux density equal to or smaller than 10 mT, a layer having coercivity equal to or smaller than 7.96 kA/m (100 Oe), or a layer having a residual magnetic flux density equal to or smaller than 10 mT and coercivity equal to or smaller than 7.96 kA/m (100 Oe). It is preferable that the non-magnetic layer does not have a residual magnetic flux density and coercivity.

Non-Magnetic Support

Next, the non-magnetic support will be described. As the non-magnetic support (hereinafter, also simply referred to as a “support”), well-known components such as polyethylene terephthalate, polyethylene naphthalate, polyamide, polyamide imide, and aromatic polyamide subjected to biaxial stretching are used. Among these, polyethylene terephthalate, polyethylene naphthalate, and polyamide are preferable. Corona discharge, plasma treatment, easy-bonding treatment, or heat treatment may be performed with respect to these supports in advance.

Back Coating Layer

The magnetic tape can also include a back coating layer including non-magnetic powder and a binding agent on a surface of the non-magnetic support opposite to the surface provided with the magnetic layer. The back coating layer preferably includes any one or both of carbon black and inorganic powder. In regards to the binding agent included in the back coating layer and various additives which can be randomly included therein, a well-known technique regarding the back coating layer can be applied, and a well-known technique regarding the list of the magnetic layer and/or the non-magnetic layer can also be applied. For example, for the back coating layer, descriptions disclosed in paragraphs 0018 to 0020 of JP2006-331625A and page 4, line 65 to page 5, line 38, of U.S. Pat. No. 7,029,774B can be referred to.

Various Thicknesses

The total thickness of the magnetic tape is equal to or smaller than 5.30 μm. A thin total thickness (thinning) is preferable for increasing a recording capacity for one reel of the magnetic tape cartridge. The total thickness of the magnetic tape, for example, may be equal to or smaller than 5.20 μm, equal to or smaller than 5.10 μm, or equal to or smaller than 5.00 μm. In addition, the total thickness of the magnetic tape, for example, is preferably equal to or greater than 1.00 μm, more preferably equal to or greater than 2.00 μm, even more preferably equal to or greater than 3.00 μm, and still even more preferably equal to or greater than 4.00 μm from a viewpoint of ease of handling (handleability) of the magnetic tape.

A thickness of the non-magnetic support of the magnetic tape is preferably 3.00 to 4.50 μm. From a viewpoint of high-density recording required in recent years, a thickness of the magnetic layer is preferably equal to or smaller than 0.15 μm and more preferably equal to or smaller than 0.10 μm. A thickness of the magnetic layer is even more preferably 0.01 to 0.10 μm. The magnetic layer may be at least one layer, or the magnetic layer can be separated into two or more layers having magnetic properties, and a configuration regarding a well-known multilayered magnetic layer can be applied. A thickness of the magnetic layer which is separated into two or more layers is a total thickness of the layers.

A thickness of the non-magnetic layer is, for example, 0.10 to 1.50 μm and preferably 0.10 to 1.00 μm.

A thickness of the back coating layer is preferably equal to or smaller than 0.90 μm and more preferably 0.10 to 0.70 μm.

The thicknesses of various layers and the non-magnetic support are obtained by exposing a cross section of the magnetic tape in a thickness direction by a well-known method of ion beams or microtome, and observing the exposed cross section with a scanning transmission electron microscope (STEM). For the specific examples of the measurement method of the thickness, a description disclosed regarding the measurement method of the thickness in examples which will be described later can be referred to.

Manufacturing Step

Preparation of Each Layer Forming Composition

Steps of preparing the composition for forming the magnetic layer, the non-magnetic layer, or the back coating layer generally include at least a kneading step, a dispersing step, and a mixing step provided before and after these steps, if necessary. Each step may be divided into two or more stages. The components used in the preparation of each layer forming composition may be added at an initial stage or in a middle stage of each step. As the solvent, one kind or two or more kinds of various solvents generally used for manufacturing a coating type magnetic recording medium can be used. For the solvent, a description disclosed in a paragraph 0153 of JP2011-216149A can be referred to, for example. In addition, each component may be separately added in two or more steps. For example, the binding agent may be separately added in the kneading step, the dispersing step, and a mixing step for adjusting a viscosity after the dispersion. In order to manufacture the magnetic tape, a well-known manufacturing technique of the related art can be used in various steps. In the kneading step, an open kneader, a continuous kneader, a pressure kneader, or a kneader having a strong kneading force such as an extruder is preferably used. For details of the kneading processes, descriptions disclosed in JP1989-106338A (JP-H01-106338A) and JP1989-079274A (JP-H01-079274A) can be referred to. As a disperser, a well-known disperser can be used. In addition, the ferromagnetic powder and the abrasive can also be dispersed separately. The separate dispersion is specifically a method of preparing a magnetic layer forming composition through a step of mixing an abrasive solution including an abrasive and a solvent (here, ferromagnetic powder is not substantially included) with a magnetic liquid including the ferromagnetic powder, a solvent, and a binding agent. The expression “ferromagnetic powder is not substantially included” means that the ferromagnetic powder is not added as a constituent component of the abrasive solution, and a small amount of the ferromagnetic powder mixed as impurities without any intention is allowed. Regarding ΔN, as a period of the dispersion time of the magnetic liquid increases, the value of ΔN tends to increase. This is considered that, as a period of the dispersion time of the magnetic liquid increases, the dispersibility of the ferromagnetic powder in the coating layer of the magnetic layer forming composition increases, and the uniformity of the alignment state of the ferromagnetic particles configuring the ferromagnetic powder by the alignment process tends to easily increase. In addition, as the period of the dispersion time in a case of mixing and dispersing various components of the non-magnetic layer forming composition increases, the value of ΔN tends to increase. The dispersion time of the magnetic liquid and the dispersion time of the non-magnetic layer forming composition may be set so that ΔN of 0.25 to 0.40 can be realized.

In any stage of preparing each layer forming composition, the filtering may be performed by a well-known method. The filtering can be performed by using a filter, for example. As the filter used in the filtering, a filter having a hole diameter of 0.01 to 3 μm (for example, filter made of glass fiber or filter made of polypropylene) can be used, for example.

Coating Step

The magnetic layer can be formed, for example, by directly applying the magnetic layer forming composition onto the non-magnetic support or performing multilayer coating of the magnetic layer forming composition with the non-magnetic layer forming composition in order or at the same time. The back coating layer can be formed by applying the back coating layer forming composition onto a side of the non-magnetic support opposite to the side provided with the magnetic layer (or magnetic layer is to be provided). In addition, the coating step for forming each layer can be also performed by being divided into two or more steps. For example, in an aspect, the magnetic layer forming composition can be applied in two or more steps. In this case, a drying process may be performed or may not be performed during the coating steps of two stages. In addition, the alignment process may be performed or may not be performed during the coating steps of two stages. For details of the coating for forming each layer, a description disclosed in a paragraph 0066 of JP2010-231843A can be referred to. In addition, for the drying step after applying the each layer forming composition, a well-known technique can be applied. Regarding the magnetic layer forming composition, as a drying temperature of a coating layer which is formed by applying the magnetic layer forming composition (hereinafter, also referred to as a “coating layer of the magnetic layer forming composition” or simply a “coating layer”) decreases, the value of ΔN tends to increase. The drying temperature can be an atmosphere temperature for performing the drying step, for example, and may be set so that ΔN of 0.25 to 0.40 can be realized.

Other Steps

For various other steps for manufacturing the magnetic tape, descriptions disclosed in paragraphs 0067 to 0070 of JP2010-231843A can be referred to.

For example, it is preferable to perform the alignment process with respect to the coating layer of the magnetic layer forming composition while the coating layer is wet. From a viewpoint of ease of exhibiting of ΔN of 0.25 to 0.40, the alignment process is preferably performed by disposing a magnet so that a magnetic field is vertically applied to the surface of the coating layer of the magnetic layer forming composition (that is, homeotropic alignment process). The strength of the magnetic field during the alignment process may be set so that ΔN of 0.25 to 0.40 can be realized. In addition, in a case of performing the coating step of the magnetic layer forming composition by the coating steps of two or more stages, it is preferable to perform the alignment process at least after the final coating step, and it is more preferable to perform the homeotropic alignment process. For example, in a case of forming the magnetic layer by the coating steps of two stages, the drying step is performed without performing the alignment process after the first coating step, and then, the alignment process can be performed with respect to the formed coating layer in the second coating step. For the alignment process, various well-known techniques such as descriptions disclosed in a paragraph 0052 of JP2010-024113A can be used. For example, the homeotropic alignment process can be performed by a well-known method such as a method using a pole opposing magnet. In the alignment zone, a drying speed of the coating layer can be controlled depending on a temperature and an air flow of dry air and/or a transportation speed of the magnetic tape in the alignment zone. In addition, the coating layer may be preliminarily dried before the transportation to the alignment zone.

In addition, the calender process can be performed in any stage after drying the coating layer of the magnetic layer forming composition. For the conditions of the calender process, a description disclosed in a paragraph 0026 of JP2010-231843A can be referred to. As the calender temperature (surface temperature of the calender roll) increases, the value of ΔN tends to increase. As the calender temperature increases, the value of the magnetic layer surface roughness Ra tends to increase. The calender temperature may be set so that ΔN of 0.25 to 0.40 and Ra of equal to or smaller than 1.8 nm can be realized.

Formation of Servo Pattern

The magnetic tape includes a servo pattern in the magnetic layer. Details of the servo pattern are as described above. For example, FIG. 1 shows an example of disposition of a region (servo band) in which the timing-based servo pattern is formed and a region (data band) interposed between two servo bands. FIG. 2 shows an example of disposition of the timing-based servo patterns. Here, the example of disposition shown in each drawing is merely an example, and the servo pattern, the servo bands, and the data bands may be disposed in the disposition according to a system of the magnetic tape apparatus (drive). In addition, for the shape and the disposition of the timing-based servo pattern, a well-known technique such as examples of disposition shown in FIG. 4, FIG. 5, FIG. 6, FIG. 9, FIG. 17, and FIG. 20 of U.S. Pat. No. 5,689,384A can be applied, for example.

The servo pattern can be formed by magnetizing a specific region of the magnetic layer by a servo write head mounted on a servo writer. The direction of magnetization can be the longitudinal direction or a vertical direction (in other words, an in-plane direction) of the magnetic tape. The formation of the servo pattern is normally formed after direct current (DC) demagnetization of the magnetic layer. The direction of demagnetization can be the longitudinal direction or the vertical direction of the magnetic tape. The region magnetized by the servo write head (position where a servo pattern is formed) is determined by standards. As the servo writer, a commercially available servo writer or a servo writer having a well-known configuration can be used. For the configuration of the servo writer, well-known techniques such as techniques disclosed in JP2011-175687A, U.S. Pat. Nos. 5,689,384A, and 6,542,325B can be referred to.

As described above, it is possible to obtain the magnetic tape according to an aspect of the invention. The magnetic tape is normally accommodated in a magnetic tape cartridge and the magnetic tape cartridge is mounted on a magnetic tape apparatus.

Magnetic Tape Cartridge

An aspect of the invention relates to a magnetic tape cartridge including the magnetic tape.

In the magnetic tape cartridge, the magnetic tape is generally accommodated in a cartridge main body in a state of being wound around a reel. The reel is rotatably provided in the cartridge main body. As the magnetic tape cartridge, a single reel type magnetic tape cartridge including one reel in a cartridge main body and a twin reel type magnetic tape cartridge including two reels in a cartridge main body are widely used. In a case where the single reel type magnetic tape cartridge is mounted in the magnetic tape apparatus (drive) in order to record and/or reproduce information (magnetic signals) on the magnetic tape, the magnetic tape is drawn from the magnetic tape cartridge and wound around the reel on the drive side. A magnetic head is disposed on a magnetic tape transportation path from the magnetic tape cartridge to a winding reel. Sending and winding of the magnetic tape are performed between a reel (supply reel) on the magnetic tape cartridge side and a reel (winding reel) on the drive side. In the meantime, the magnetic head comes into contact with and slides on the surface of the magnetic layer of the magnetic tape, and accordingly, the recording and/or reproduction of information is performed. With respect to this, in the twin reel type magnetic tape cartridge, both reels of the supply reel and the winding reel are provided in the magnetic tape cartridge. The magnetic tape cartridge may be any of single reel type magnetic tape cartridge and twin reel type magnetic tape cartridge. The magnetic tape cartridge may include the magnetic tape according to an aspect of the invention, and a well-known technique can be applied for other configurations.

Magnetic Tape Apparatus

An aspect of the invention relates to a magnetic tape apparatus including the magnetic tape and a magnetic head.

In the invention and the specification, the “magnetic tape apparatus” means a device capable of performing at least one of the recording of information on the magnetic tape or the reproducing of information recorded on the magnetic tape. Such an apparatus is generally called a drive. The magnetic tape apparatus can be a sliding type magnetic tape apparatus. The sliding type apparatus is an apparatus in which the surface of the magnetic layer comes into contact with and slides on the magnetic head, in a case of performing the recording of information on the magnetic tape and/or reproducing of the recorded information.

The magnetic head included in the magnetic tape apparatus can be a recording head capable of performing the recording of information on the magnetic tape, or can be a reproducing head capable of performing the reproducing of information recorded on the magnetic tape. In addition, in an aspect, the magnetic tape apparatus can include both of a recording head and a reproducing head as separate magnetic heads. In another aspect, the magnetic head included in the magnetic tape can also have a configuration of comprising both of a recording element and a reproducing element in one magnetic head. As the reproducing head, a magnetic head (MR head) including a magnetoresistive (MR) element capable of sensitively reading information recorded on the magnetic tape as a reproducing element is preferable. As the MR head, various well-known MR heads can be used. In addition, the magnetic head which performs the recording of information and/or the reproducing of information may include a servo pattern reading element. Alternatively, as a head other than the magnetic head which performs the recording of information and/or the reproducing of information, a magnetic head (servo head) comprising a servo pattern reading element may be included in the magnetic tape apparatus.

The details of the magnetic tape mounted on the magnetic tape apparatus are as described above. Such a magnetic tape includes servo patterns. Accordingly, a magnetic signal is recorded on the data band by the magnetic head to form a data track, and/or, in a case of reproducing the recorded signal, head tracking is performed based on the read servo pattern, while reading the servo pattern by the servo head, and therefore, it is possible to cause the magnetic head to follow the data track at a high accuracy.

For details of the head tracking servo of the timing-based servo system, for example, well-known techniques such as techniques disclosed in U.S. Pat. Nos. 5,689,384A, 6,542,325B, and U.S. Pat. No. 7,876,521B can be used. In addition, for the details of the head tracking servo in the amplitude-based servo system, well-known techniques disclosed in U.S. Pat. Nos. 5,426,543A and 5,898,533A can be used.

A commercially available magnetic tape apparatus generally includes a magnetic head in accordance to a standard. In addition, a commercially available magnetic tape apparatus generally has a servo controlling mechanism for realizing head tracking of the servo system in accordance to a standard. The magnetic tape apparatus according to an aspect of the invention can be configured by incorporating the magnetic tape according to an aspect of the invention to a commercially available magnetic tape apparatus.

EXAMPLES

Hereinafter, the invention will be described with reference to examples. However, the invention is not limited to aspects shown in the examples. “Parts” and “%” in the following description mean “parts by mass” and “% by mass”, unless otherwise noted. In addition, steps and evaluations described below are performed in an environment of an atmosphere temperature of 23° C.±1° C., unless otherwise noted.

Example 1

Preparation of Abrasive Solution 3.0 parts of 2,3-dihydroxynaphthalene (manufactured by Tokyo Chemical Industry Co., Ltd.), 31.3 parts of a 32% solution (solvent is a mixed solvent of methyl ethyl ketone and toluene) of a SO₃Na group-containing polyester polyurethane resin (UR-4800 (SO₃Na group: 0.08 meq/g) manufactured by Toyobo Co., Ltd.), and 570.0 parts of a mixed solvent of methyl ethyl ketone and cyclohexanone (mass ratio of 1:1) as a solvent were mixed with 100.0 parts of alumina powder (HIT-80 manufactured by Sumitomo Chemical Co., Ltd.) having a gelatinization ratio of approximately 65% and a BET specific surface area of 20 m²/g, and dispersed in the presence of zirconia beads by a paint shaker for 5 hours. After the dispersion, the dispersion liquid and the beads were separated by a mesh and an alumina dispersion was obtained.

Preparation of Magnetic Layer Forming Composition

Magnetic Liquid Plate-shaped ferromagnetic hexagonal 100.0 parts barium ferrite powder activation volume: 1,600 nm³, average plate ratio: 3.5 SO₃Na group-containing polyurethane resin see Table 1 Weight-average molecular weight: 70,000, SO₃Na group: see Table 1 Cyclohexanone 150.0 parts Methyl ethyl ketone 150.0 parts Abrasive Solution Alumina dispersion prepared as described above 6.0 parts Silica Sol (projection forming agent liquid) Colloidal silica (average particle size: 100 nm) 2.0 parts Methyl ethyl ketone 1.4 parts Other Components Stearic acid 2.0 parts Butyl stearate 2.0 parts Polyisocyanate (CORONATE (registered trademark) 2.5 parts manufactured by Tosoh Corporation) Finishing Additive Solvent Cyclohexanone 200.0 parts Methyl ethyl ketone 200.0 parts

Preparation Method

The magnetic liquid was prepared by beads-dispersing of various components of the magnetic liquid described above by using beads as the dispersion medium in a batch type vertical sand mill. The bead dispersion was performed using zirconia beads (bead diameter: see Table 1) as the beads for the time shown in Table 1 (magnetic liquid bead dispersion time).

The magnetic liquid obtained as described above, the abrasive solution, silica sol, other components, and a finishing additive solvent were mixed with each other and beads-dispersed for 5 minutes, and the treatment (ultrasonic dispersion) was performed with a batch type ultrasonic device (20 kHz, 300 W) for 0.5 minutes. After that, the obtained mixed solution was filtered by using a filter having a hole diameter of 0.5 μm, and the magnetic layer forming composition was prepared.

Preparation of Non-Magnetic Layer Forming Composition

Each component among various components of the non-magnetic layer forming composition shown below excluding stearic acid, butyl stearate, cyclohexanone, and methyl ethyl ketone was beads-dispersed (dispersion medium: zirconia beads (bead diameter: 0.1 mm), dispersion time: see Table 1) by using a batch type vertical sand mill to obtain a dispersion liquid. After that, the remaining components were added into the obtained dispersion liquid and stirred with a dissolver. Then, the obtained dispersion liquid was filtered with a filter (hole diameter: 0.5 μm) and a non-magnetic layer forming composition was prepared.

Non-magnetic inorganic powder: α-iron oxide 100.0 parts Average particle size (average major axis length): 0.15 μm Average acicular ratio: 7 BET specific surface area: 52 m²/g Carbon black 20.0 parts Average particle size: 20 nm Electron beam curable vinyl chloride copolymer 13.0 parts Electron beam curable polyurethane resin 6.0 parts Stearic acid 1.0 part Butyl stearate 1.0 part Cyclohexanone 300.0 parts Methyl ethyl ketone 300.0 parts

Preparation of Back Coating Layer Forming Composition

Each component among various components of the back coating layer forming composition shown below excluding stearic acid, butyl stearate, polyisocyanate, and cyclohexanone was kneaded and diluted by an open kneader, and a mixed solution was obtained. After that, the obtained mixed solution was subjected to a dispersion process of 12 passes, with a transverse beads mill and zirconia beads having a bead diameter of 1.0 mm, by setting a bead filling percentage as 80 volume %, a circumferential speed of rotor distal end as 10 m/sec, and a retention time for 1 pass as 2 minutes. After that, the remaining components were added into the obtained dispersion liquid and stirred with a dissolver. Then, the obtained dispersion liquid was filtered with a filter (hole diameter: 1.0 nm) and a back coating layer forming composition was prepared.

Non-magnetic inorganic powder: α-iron oxide 80.0 parts Average particle size (average major axis length): 0.15 μm Average acicular ratio: 7 BET specific surface area: 52 m²/g Carbon black 20.0 parts Average particle size: 20 nm A vinyl chloride copolymer 13.0 parts A sulfonic acid salt group- 6.0 parts containing polyurethane resin Phenylphosphonic acid 3.0 parts Methyl ethyl ketone 155.0 parts Stearic acid 3.0 parts Butyl stearate 3.0 parts Polyisocyanate 5.0 parts Cyclohexanone 355.0 parts

Manufacturing of Magnetic Tape

The non-magnetic layer forming composition was applied and dried onto a polyethylene naphthalate support, and then, an electron beam was emitted with an energy of 40 kGy at an acceleration voltage of 125 kV, to form a non-magnetic layer.

The magnetic layer forming composition was applied on a surface of the formed non-magnetic layer to form a coating layer. A homeotropic alignment process and a drying process were performed by applying a magnetic field having a strength shown in a column of “formation and alignment of magnetic layer” of Table 1 to the surface of the coating layer in a vertical direction by using a pole opposing magnet in an atmosphere at an atmosphere temperature (magnetic layer drying temperature) shown in Table 1, while this coating layer was wet, and a magnetic layer was formed.

After that, the back coating layer forming composition was applied and dried on a surface of the support on a side opposite to the surface on which the non-magnetic layer and the magnetic layer were formed.

Then, a surface smoothing treatment (calender process) was performed with a calender roll configured of only a metal roll, at a calender process speed of 80 m/min, linear pressure of 300 kg/cm (294 kN/m), and a calender temperature (surface temperature of a calender roll) shown in Table 1.

Then, the thermal treatment was performed in the environment of the atmosphere temperature of 70° C. for 36 hours. After the thermal treatment, the slitting was performed so as to have a width of ½ inches (1 inch is 0.0254 meters), and the surface of the magnetic layer was cleaned with a tape cleaning device in which a nonwoven fabric and a razor blade are attached to a device including a sending and winding device of the slit so as to press the surface of the magnetic layer. Then, servo patterns (timing-based servo patterns) having disposition and shapes according to the LTO Ultrium format were formed on the magnetic layer by a commercially available servo writer.

As described above, the magnetic tape of Example 1 was manufactured.

Examples 2 and 4, Comparative Examples 1 to 4 and 6, and Reference Examples 1 to 4

A magnetic tape was manufactured by the same method as that in Example 1, except that various conditions shown in Table 1 were changed as shown in Table 1. The thickness of each layer was adjusted by the coating amount of the each layer forming composition.

In Table 1, in the comparative examples and the reference examples in which “no alignment process” is shown in the column of “formation and alignment of magnetic layer”, the magnetic tape was manufactured without performing the alignment process regarding the coating layer of the magnetic layer forming composition.

Example 3

After forming the non-magnetic layer on the polyethylene naphthalate support in the same manner as in Example 1, the magnetic layer forming composition was applied on the surface of the non-magnetic layer so that the thickness after drying becomes 50 nm to form a first coating layer. The first coating layer was passed through the atmosphere at the atmosphere temperature shown in Table 1 (magnetic layer drying temperature) without application of a magnetic field to form a first magnetic layer (no alignment process).

After that, the magnetic layer forming composition was applied on the surface of the first magnetic layer so that the thickness after drying becomes 50 nm to form a second coating layer. A homeotropic alignment process and a drying process were performed by applying a magnetic field having a strength shown in the column of “formation and alignment of magnetic layer” of Table 1 to the surface of the second coating layer in a vertical direction by using a pole opposing magnet in an atmosphere at an atmosphere temperature (magnetic layer drying temperature) shown in Table 1, while this second coating layer was wet, and a second magnetic layer was formed.

A magnetic tape was manufactured by the same method as that in Example 1, except that the multilayered magnetic layer was formed as described above.

Comparative Example 5

After forming the non-magnetic layer on the polyethylene naphthalate support in the same manner as in Example 1, the magnetic layer forming composition was applied on the surface of the non-magnetic layer so that the thickness after drying becomes 50 nm to form a first coating layer. The homeotropic alignment process and the drying process were performed by applying a magnetic field having a strength shown in the column of “formation and alignment of magnetic layer” of Table 1 to the surface of the first coating layer in a vertical direction by using a pole opposing magnet in an atmosphere at an atmosphere temperature (magnetic layer drying temperature) shown in Table 1, while this first coating layer was wet, and a first magnetic layer was formed.

After that, the magnetic layer forming composition was applied on the surface of the first magnetic layer so that the thickness after drying becomes 50 nm to form a second coating layer. The second coating layer was passed through the atmosphere at the atmosphere temperature shown in Table 1 (magnetic layer drying temperature) without application of a magnetic field to form a second magnetic layer (no alignment process).

A magnetic tape was manufactured by the same method as that in Example 1, except that the multilayered magnetic layer was formed as described above.

Measurement Method

(1) Magnetic Layer Surface Roughness Ra

A measurement area of 40 μm×40 μm on the surface of the magnetic layer of the magnetic tape was measured by using an atomic force microscope (AFM, Nanoscope 4 manufactured by Veeco Instruments Inc.) in a tapping mode, and a center line average surface roughness Ra (magnetic layer surface roughness Ra) was obtained. RTESP-300 manufactured by Bruker Japan K.K. was used as a probe, a scan speed (probe moving speed) was 40 μm/sec, and a resolution was 512 pixels×512 pixels.

(2) Thicknesses of Non-Magnetic Support and Each Layer

The thicknesses of the magnetic layer, the non-magnetic layer, the non-magnetic support, and the back coating layer of each manufactured magnetic tape were measured by the following method. Table 1 shows the measured various thicknesses and the total magnetic tape thickness calculated from the various thicknesses.

The thicknesses of the magnetic layer, the non-magnetic layer, and the non-magnetic support measured here were used for calculating the following refractive index.

(i) Manufacturing of Cross Section Observation Sample

A cross section observation sample including the entire region in the thickness direction from the magnetic layer side surface of the magnetic tape to the back coating layer side surface was manufactured according to the method disclosed in paragraphs 0193 and 0194 of JP2016-177851A.

(ii) Thickness Measurement

The manufactured sample was observed with the STEM and a STEM image was captured. This STEM image was a STEM-high-angle annular dark field (HAADF) image which is captured at an acceleration voltage of 300 kV and a magnification ratio of imaging of 450,000, and imaging was performed so that the entire region in the thickness direction from the magnetic layer side surface of the magnetic tape to the back coating layer side surface was included in one image. In the STEM image obtained as described above, a linear line connecting both ends of a line segment showing the surface of the magnetic layer to each other was determined as a reference line showing the surface of the magnetic tape on the magnetic layer side. In a case where the STEM image was captured so that the magnetic layer side of the cross section observation sample was positioned on the upper side of the image and the back coating layer side was positioned on the lower side, for example, the linear line connecting both ends of the line segment described above to each other is a linear line connecting an intersection between a left side of the image (shape is a rectangular or square shape) of the STEM image and the line segment, and an intersection between a right side of the STEM image and the line segment to each other. In the same manner as described above, a reference line showing the interface between the magnetic layer and the non-magnetic layer, a reference line showing the interface between the non-magnetic layer and the non-magnetic support, a reference line showing the interface between the non-magnetic support and the back coating layer, and a reference line showing the surface of the magnetic tape on the back coating layer side were determined.

The thickness of the magnetic layer was obtained as the shortest distance from one position randomly selected on the reference line showing the surface of the magnetic tape on the magnetic layer side to the reference line showing the interface between the magnetic layer and the non-magnetic layer. In the same manner as described above, the thicknesses of the non-magnetic layer, the non-magnetic support, and the back coating layer were obtained.

(3) ΔN of Magnetic Layer

Hereinafter, M-2000U manufactured by J. A. Woollam Co. was used as an ellipsometer. Creation and fitting of the double-layer model or the single-layer model were performed using WVASE32 manufactured by J. A. Woollam Co. as analysis software.

(i) Measurement Refractive Index of Non-Magnetic Support

A sample for measurement was cut out from each magnetic tape. The cloth not used was permeated with fresh methyl ethyl ketone, the back coating layer of the sample for measurement was wiped off and removed using this cloth to expose the surface of the non-magnetic support, and then, this surface is roughened with sand paper so that reflected light of the exposed surface is not detected in the measurement which will be performed after this with an ellipsometer.

After that, by causing the cloth to permeate with fresh methyl ethyl ketone, by wiping off and removing the magnetic layer and the non-magnetic layer of the sample for measurement using the cloth and bonding a surface of a silicon wafer and the roughened surface to each other using static electricity, the sample for measurement was disposed on the silicon wafer so that the surface of the non-magnetic support exposed by removing the magnetic layer and the non-magnetic layer (hereinafter, referred to as the “surface of the non-magnetic support on the magnetic layer side”) faced upward.

An incidence ray was incident to the surface of the non-magnetic support of the sample for measurement on the magnetic layer side on the silicon wafer using an ellipsometer as described above, to measure D and y. By using the obtained measurement values and the thickness of the non-magnetic support obtained in the section (2), the refractive indexes of the non-magnetic support (the refractive index in a longitudinal direction, the refractive index in a width direction, the refractive index in a thickness direction measured by incidence of incidence ray in the longitudinal direction, and the refractive index in a thickness direction measured by incidence of incidence ray in the width direction) were obtained by the method described above.

(ii) Measurement of Refractive Index of Non-Magnetic Layer

A sample for measurement was cut out from each magnetic tape. The cloth not used was permeated with methyl ethyl ketone, the back coating layer of the sample for measurement was wiped off and removed using this cloth to expose the surface of the non-magnetic support, and then, this surface is roughened with sand paper so that reflected light of the exposed surface is not detected in the measurement which will be performed after this with the spectroscopic ellipsometer.

After that, the cloth not used was permeated with fresh methyl ethyl ketone, the surface of the magnetic layer of the sample for measurement was wiped off using this cloth, the magnetic layer was removed to expose the surface of the non-magnetic layer, and then, the sample for measurement was disposed on the silicon wafer in the same manner as in the section (i).

The measurement regarding the surface of the non-magnetic layer of the sample for measurement on the silicon wafer was performed using an ellipsometer, and the refractive indexes of the non-magnetic layer (the refractive index in a longitudinal direction, the refractive index in a width direction, the refractive index in a thickness direction measured by incidence of incidence ray in the longitudinal direction, and the refractive index in a thickness direction measured by incidence of incidence ray in the width direction) were obtained by the method described above in a spectral ellipsometry.

(iii) Measurement of Refractive Index of Magnetic Layer

A sample for measurement was cut out from each magnetic tape. The cloth not used was permeated with fresh methyl ethyl ketone, the back coating layer of the sample for measurement was wiped off and removed using this cloth to expose the surface of the non-magnetic support, and then, this surface is roughened with sand paper so that reflected light of the exposed surface is not detected in the measurement which will be performed after this with the spectroscopic ellipsometer.

After that, the sample for measurement was disposed on the sample for measurement on the silicon wafer, in the same manner as in the section (i).

The measurement regarding the surface of the magnetic layer of the sample for measurement on the silicon wafer was performed using an ellipsometer, and the refractive indexes of the magnetic layer (the refractive index Nx in a longitudinal direction, the refractive index Ny in a width direction, the refractive index Nz₁ in a thickness direction measured by incidence of incidence ray in the longitudinal direction, and the refractive index Nz₂ in a thickness direction measured by incidence of incidence ray in the width direction) were obtained by the method described above in a spectral ellipsometry. Nxy and Nz were obtained from the obtained values, and the absolute value ΔN of the difference of these values was obtained. Regarding all of magnetic tapes of the examples, the comparative examples, and the reference examples, the obtained Nxy was a value greater than Nz (that is, Nxy>Nz).

(4) Vertical Squareness Ratio (SQ)

A vertical squareness ratio of the magnetic tape is a squareness ratio measured in a vertical direction of the magnetic tape. The “vertical direction” described regarding the squareness ratio is a direction orthogonal to the surface of the magnetic layer. Regarding each magnetic tape of the examples, the comparative examples, and the reference examples, the vertical squareness ratio was obtained by sweeping an external magnetic field in the magnetic tape at a measurement temperature of 23° C.±1° C. using an vibrating sample magnetometer (manufactured by Toei Industry Co., Ltd.) under conditions of a maximum external magnetic field of 1194 kA/m (15 kOe) and a scan speed of 4.8 kA/m/sec (60 Oe/sec). The measurement value is a value after diamagnetic field correction, and is obtained as a value obtained by subtracting magnetization of a sample probe of the vibrating sample magnetometer as background noise. In an aspect, the vertical squareness ratio of the magnetic tape is preferably 0.60 to 1.00. In addition, in an aspect, the vertical squareness ratio of the magnetic tape can be, for example, equal to or smaller than 0.90, equal to or smaller than 0.85, or equal to or smaller than 0.80, and can also be greater than these values.

(5) Occurrence Frequency of Signal Defect (Thermal Asperity) at Time of Reproducing Servo Signal

The magnetic tapes of the examples, the comparative examples, and the reference examples were attached to a servo tester. In the servo tester, the reading of the servo patterns (reproducing of servo signals) was performed by the servo head by allowing each magnetic tape to run and the surface of the magnetic layer of the running magnetic tape and the servo head on which the MR element is mounted to come into contact with each other and sliding. In a reproduced waveform of the servo signal obtained by the reproduction, a part that is not a normal burst signal and shows an output of equal to or greater than 200% with an average value of a noise level output as 100% is determined as thermal asperity, and the number of thermal asperities was counted. A value (number of times/m) obtained by dividing the counted number of thermal asperities by total length of the magnetic tape is defined as the occurrence frequency of the signal defect (thermal asperity).

The results are shown in Table 1 (Table 1-1 to Table 1-4).

TABLE 1-1 Example 1 Example 2 Example 3 Example 4 Magnetic layer thickness (μm) 0.10 0.10 0.10 0.10 Non-magnetic layer thickness 0.70 0.50 0.70 0.70 (gm) Non-magnetic support thickness 4.20 4.00 4.20 4.20 (μm) Back coating layer thickness 0.30 0.30 0.30 0.30 (μm) Magnetic tape total thickness 5.30 4.90 5.30 5.30 (μm) Magnetic liquid bead dispersion 50 hours 50 hours 50 hours 50 hours time Magnetic liquid dispersion bead 0.1 mm 0.1 mm 0.1 mm 0.1 mm diameter Magnetic liquid 330 eq/ton 330 eq/ton 330 eq/ton 330 eq/ton Content of SO₃Na group of polyurethane resin Magnetic liquid 15.0 parts 15.0 parts 15.0 parts 15.0 parts Content of SO₃Na group-containing polyurethane resin Non-magnetic layer forming 24 hours 24 hours 24 hours 24 hours composition dispersion time Magnetic layer drying 50° C. 50° C. 50° C. 50° C. temperature Calender temperature 100° C. 100° C. 100° C. 100° C. Formation and alignment of Homeotropic Homeotropic Second magnetic Homeotropic magnetic layer alignment 0.5 T alignment 0.5 T layer: homeotropic alignment 0.2 T alignment 0.5 T/ First magnetic layer: no alignment process Magnetic layer surface 1.8  1.8  1.8  1.8  roughness Ra (nm) Vertical squareness ratio (SQ) 0.66 0.66 0.60 0.60 Nxy 1.90 1.90 1.95 1.90 Nz 1.60 1.60 1.60 1.65 ΔN 0.30 0.30 0.35 0.25 Occurrence frequency of signal 0.10 0.15 0.10 0.05 defect (thermal asperity) (number of times/m)

TABLE 1-2 Comparative Comparative Comparative Example 1 Example 2 Example 3 Magnetic layer thickness (μm) 0.10 0.10 0.10 Non-magnetic layer thickness 0.70 0.50 0.50 (μm) Non-magnetic support thickness 4.20 4.00 4.00 (μm) Back coating layer thickness 0.30 0.30 0.30 (μm) Magnetic tape total thickness 5.30 4.90 4.90 (μm) Magnetic liquid bead dispersion 6 hours 6 hours 6 hours time Magnetic liquid dispersion bead 1.0 mm 1.0 mm 1.0 mm diameter Magnetic liquid 60 eq/ton 60 eq/ton 60 eq/ton Content of SO₃Na group of polyurethane resin Magnetic liquid 25.0 parts 25.0 parts 25.0 parts Content of SO₃Na group-containing polyurethane resin Non-magnetic layer forming 3 hours 3 hours 3 hours composition dispersion time Magnetic layer drying 70° C. 70° C. 70° C. temperature Calender temperature 100° C. 100° C. 105° C. Formation and alignment of No alignment process No alignment process No alignment process magnetic layer Magnetic layer surface 1.8  1.8  1.6  roughness Ra (nm) Vertical squareness ratio (SQ) 0.50 0.50 0.50 Nxy 1.90 1.90 1.90 Nz 1.80 1.80 1.80 ΔN 0.10 0.10 0.10 Occurrence frequency of signal 1.00 1.50 7.50 defect (thermal asperity) (number of times/m)

TABLE 1-3 Comparative Comparative Comparative Example 4 Example 5 Example 6 Magnetic layer thickness (μm) 0.10 0.10 0.10 Non-magnetic layer thickness 0.70 0.70 0.70 (μm) Non-magnetic support thickness 4.20 4.20 4.20 (μm) Back coating layer thickness 0.30 0.30 0.30 (μm) Magnetic tape total thickness 5.30 5.30 5.30 (μm) Magnetic liquid bead dispersion 50 hours 50 hours 96 hours time Magnetic liquid dispersion bead 0.1 mm 0.1 mm 0.1 mm diameter Magnetic liquid 330 eq/ton 330 eq/ton 330 eq/ton Content of SO₃Na group of polyurethane resin Magnetic liquid 15.0 parts 15.0 parts 10.0 parts Content of SO₃Na group-containing polyurethane resin Non-magnetic layer forming 24 hours 24 hours 48 hours composition dispersion time Magnetic layer drying 50° C. 50° C. 30° C. temperature Calender temperature 100° C. 100° C. 110° C. Formation and alignment of No alignment process Second magnetic Homeotropic magnetic layer layer: alignment 0.5 T no alignment process/ First magnetic layer: homeotropic alignment 0.5 T Magnetic layer surface 1.8  1.8  1.5  roughness Ra (nm) Vertical squareness ratio (SQ) 0.53 0.60 0.80 Nxy 1.90 1.90 2.20 Nz 1.70 1.70 1.75 ΔN 0.20 0.20 0.45 Occurrence frequency of signal 8.00 10.00  13.00  defect (thermal asperity) (number of times/m)

TABLE 1-4 Reference Reference Reference Reference Example 1 Example 2 Example 3 Example 4 Magnetic layer thickness (μm) 0.10 0.10 0.10 0.10 Non-magnetic layer thickness 1.00 0.70 0.70 0.50 (μm) Non-magnetic support thickness 4.30 4.20 4.20 4.00 (μm) Back coating layer thickness 0.60 0.40 0.30 0.30 (μm) Magnetic tape total thickness 6.00 5.40 5.30 4.90 (μm) Magnetic liquid bead dispersion 6 hours 6 hours 6 hours 6 hours time Magnetic liquid dispersion bead 1.0 mm 1.0 mm 1.0 mm 1.0 mm diameter Magnetic liquid 60 eq/ton 60 eq/ton 60 eq/ton 60 eq/ton Content of SO₃Na group of polyurethane resin Magnetic liquid 25.0 parts 25.0 parts 25.0 parts 25.0 parts Content of SO₃Na group-containing polyurethane resin Non-magnetic layer forming 3 hours 3 hours 3 hours 3 hours composition dispersion time Magnetic layer drying 70° C. 70° C. 70° C. 70° C. temperature Calender temperature 100° C. 100° C. 90° C. 90° C. Formation and alignment of No alignment No alignment No alignment No alignment magnetic layer process process process process Magnetic layer surface 1.8  1.8  2.2  2.2  roughness Ra (nm) Vertical squareness ratio (SQ) 0.50 0.50 0.50 0.50 Nxy 1.90 1.90 1.90 1.90 Nz 1.80 1.80 1.80 1.80 ΔN 0.10 0.10 0.10 0.10 Occurrence frequency of signal 0.08 0.10 0.05 0.12 defect (thermal asperity) (number of times/m)

By comparison of Reference Examples 1 to 4 and Comparative Examples 1 to 6 with each other, in a case where the total thickness of the magnetic tape is equal to or smaller than 5.30 μm and the magnetic layer surface roughness Ra is equal to or smaller than 1.8 nm compared to a case where the total thickness of the magnetic tape is greater than 5.30 μm (Reference Examples 1 and 2) and a case where the magnetic layer surface roughness Ra is greater than 1.8 nm (Reference Examples 3 and 4), it was confirmed that the occurrence frequency of the signal defect significantly increases at the time of reproducing the servo signal (Comparative Examples 1 to 6).

With respect to this, the magnetic tapes of Examples 1 to 4 have the total thickness of equal to or smaller than 5.30 μm and the magnetic layer surface roughness Ra of equal to or smaller than 1.8 nm, but the occurrence frequency of the signal defect was greatly reduced at the time of reproducing the servo signal compared to the magnetic tapes of Comparative Examples 1 to 6.

In general, the squareness ratio is known as an index for a state of the ferromagnetic powder present in the magnetic layer. However, as shown in Table 1, even in a case of the magnetic tapes having the same vertical squareness ratios, ΔN's are different from each other (for example, Examples 3 and 4 and Comparative Example 5). The inventor has considered that this shows that ΔN is a value which is affected by other factors, in addition to the state of the ferromagnetic powder present in the magnetic layer.

An aspect of the invention is effective in a technical field of magnetic tapes for high-density recording. 

What is claimed is:
 1. A magnetic tape comprising: a non-magnetic support; a magnetic layer including ferromagnetic powder on the non-magnetic support; and a non-magnetic layer including non-magnetic powder between the non-magnetic support and the magnetic layer, wherein a total thickness of the magnetic tape is equal to or smaller than 5.30 μm, a thickness of the non-magnetic layer is 0.10 nm to 1.50 μm, the magnetic layer has a servo pattern, a center line average surface roughness Ra measured on a surface of the magnetic layer is equal to or smaller than 1.8 nm, and an absolute value ΔN of a difference between a refractive index Nxy of the magnetic layer, measured in an in-plane direction and a refractive index Nz of the magnetic layer, measured in a thickness direction is 0.25 to 0.40.
 2. The magnetic tape according to claim 1, wherein a difference Nxy−Nz between the refractive index Nxy and the refractive index Nz is 0.25 to 0.40.
 3. The magnetic tape according to claim 1, wherein the total thickness of the magnetic tape is 3.00 μm to 5.30 μm.
 4. The magnetic tape according to claim 1, further comprising: a back coating layer including non-magnetic powder on a surface of the non-magnetic support opposite to a surface provided with the magnetic layer.
 5. The magnetic tape according to claim 1, wherein the center line average surface roughness Ra measured on the surface of the magnetic layer is 1.2 nm to 1.8 nm.
 6. The magnetic tape according to claim 1, wherein the servo pattern is a timing-based servo pattern.
 7. The magnetic tape according to claim 1, wherein the thickness of the non-magnetic layer is 0.10 μm to 1.00 μm.
 8. The magnetic tape according to claim 1, wherein the thickness of the non-magnetic layer is 0.10 μm to 0.70 μm.
 9. The magnetic tape according to claim 1, wherein the thickness of the non-magnetic layer is 0.10 μm to 0.50 μm.
 10. A magnetic tape cartridge comprising: a magnetic tape, which comprises: a non-magnetic support; a magnetic layer including ferromagnetic powder on the non-magnetic support; and a non-magnetic layer including non-magnetic powder between the non-magnetic support and the magnetic layer, wherein a total thickness of the magnetic tape is equal to or smaller than 5.30 μm, a thickness of the non-magnetic layer is 0.10 μm to 1.50 μm, the magnetic layer has a servo pattern, a center line average surface roughness Ra measured on a surface of the magnetic layer is equal to or smaller than 1.8 nm, and an absolute value ΔN of a difference between a refractive index Nxy of the magnetic layer, measured in an in-plane direction and a refractive index Nz of the magnetic layer, measured in a thickness direction is 0.25 to 0.40.
 11. The magnetic tape cartridge according to claim 10, wherein a difference Nxy−Nz between the refractive index Nxy and the refractive index Nz is 0.25 to 0.40.
 12. The magnetic tape cartridge according to claim 10, wherein the magnetic tape further comprises: a back coating layer including non-magnetic powder and on a surface of the non-magnetic support opposite to a surface provided with the magnetic layer.
 13. The magnetic tape cartridge according to claim 10, wherein the thickness of the non-magnetic layer is 0.10 μm to 1.00 μm.
 14. The magnetic tape cartridge according to claim 10, wherein the thickness of the non-magnetic layer is 0.10 μm to 0.70 μm.
 15. The magnetic tape cartridge according to claim 10, wherein the thickness of the non-magnetic layer is 0.10 μm to 0.50 μm.
 16. A magnetic tape apparatus comprising: a magnetic head; and a magnetic tape, which comprises: a non-magnetic support; a magnetic layer including ferromagnetic powder and on the non-magnetic support; and a non-magnetic layer including non-magnetic powder between the non-magnetic support and the magnetic layer, wherein a total thickness of the magnetic tape is equal to or smaller than 5.30 μm, a thickness of the non-magnetic layer is 0.10 μm to 1.50 μm, the magnetic layer has a servo pattern, a center line average surface roughness Ra measured on a surface of the magnetic layer is equal to or smaller than 1.8 nm, and an absolute value ΔN of a difference between a refractive index Nxy of the magnetic layer, measured in an in-plane direction and a refractive index Nz of the magnetic layer, measured in a thickness direction is 0.25 to 0.40.
 17. The magnetic tape apparatus according to claim 16, wherein a difference Nxy−Nz between the refractive index Nxy and the refractive index Nz is 0.25 to 0.40.
 18. The magnetic tape apparatus according to claim 16, wherein the thickness of the non-magnetic layer is 0.10 μm to 1.00 μm.
 19. The magnetic tape apparatus according to claim 16, wherein the thickness of the non-magnetic layer is 0.10 μm to 0.70 μm.
 20. The magnetic tape apparatus according to claim 16, wherein the thickness of the non-magnetic layer is 0.10 μm to 0.50 μm. 